The X field is a mature oil field producing with water injection in place. As part of a phased development, in Phase Two, two infill wells were planned and drilled to extract incremental recovery from a discovered undeveloped reservoir. This includes planning two horizontal producer wells requiring active real time geosteering utilizing deep resistivity tool technology. The wells’ objectives are to ensure the well placement was in an optimal location and to maintain the trajectory within a reservoir that is approximately 100ft thick and was believed to be homogenous field wide. The main challenge to this feat is the faulted nature of the field and the uncertainty in reservoir thickness and extend due to limited well penetrations at this reservoir level.
During the planning phase, it was identified early that a deep resistivity tool would be beneficial in geosteering the wells. Prior to drilling, an integrated pre-job model was designed to test multiple tool settings and subsurface scenarios to strategize an execution plan identifying key points where there is a need for real time trajectory adjustments and to pre-plan alternative trajectories based on subsurface scenarios to enable efficient turnaround time to react to real-time results.
Conventional navigation tools yield only a shallow to medium depth of measurement (~15ft) which would not have met the objectives of the well given the geological complexities (high fault offsets, laminated reservoirs) and well design (high angle to horizontal). The ultra-deep resistivity (UDR) tool was employed instead to enable trajectory optimization with up to ~100ft depth of investigation (DOI), using a multi-frequency, multi-spaced antenna design from medium and long spaced transmitter receiver spacings providing up to 9 vector components. In real time, the 1D inversion (using 5 of the vector components) was used for early sand and fluid contact detection.
During execution, the same integrated team was monitoring the well and close interaction between the subsurface, geosteering and directional drilling team was a key requirement to ensure drilling of the well was safely and objectively executed, especially with the challenges posed with virtual working through a pandemic.
As is when dealing with subsurface uncertainties, there were numerous surprises encountered during the drilling of the horizontal wells. Particularly in the matter of fault throw uncertainty and sand distribution. The initial 1D real-time UDR results were able to assist in real-time trajectory adjustments and to provide some geological understandings with regards to fault throw and location of possible faults along the well bore which were then confirmed with borehole image logs. Additionally, 3D inversion images were processed post drilling, and further geological insights were discovered with regards to the depositional trends on the reservoir. In a reservoir that was initially thought to be sand-rich and homogenous, 3D inversion suggests evidence of possible channels. This revelation could explain the varying thickness of the reservoir that was observed during drilling on the 1D UDR canvass.
There are plans for future work to incorporate the observations and the analysis of the UDR products for deeper reservoir understanding of the field. Studies to include full integration with seismic data and production data would prove beneficial in well and reservoir management. Additionally, insights gleaned from the optimized selection of tool frequency for real time use and calibration with azimuthal dips and images proved invaluable especially in resolving unexpected structural and depositional complexities. The challenges in delineating fluid contacts in a structurally complex reservoir was also apparent with multiple realizations (and associated probabilities) of contacts seen from the real time results, which proved valuable in re-affirming the difficulties in characterizing the uncertainties in the field